Martensitic stainless steels defined as SUS 410, SUS420 and others in JIS (Japanese Industrial Standards) have high strength and excellent corrosion resistance even in a corrosive environment containing CO.sub.2, and thereby have been used as materials for seamless steel pipes, such as oil well pipes, line pipes, geothermal well pipes and others.
The seamless steel pipe is generally produced by means of an inclined rolling method, such as Mannesmann plug mill process and Mannesmann mandrel mill process, a hot extrusion method such as Ugine-Sejournet process, or a hot press method such as Erhart pushbench process. It has been known that reducing S (sulfur) in addition to keeping down Cr equivalent [Cr+4Si-(22C+0.5Mn+1.5Ni+30N)] of a steel was desirable to prevent surface defects, such as cracks and scabs (or laps), which are likely to result from these hot workings.
In oil well pipes and the like, it is often the case that each of the pipes is provided with connecting screws at both ends. The martensitic stainless steel originally has a large cutting resistance, and the steel, having the reduced S content as described above, is likely to experience a seizure between a cutting tool and a cutting work in the same manner as austenitic stainless steels. This results in a shortened life of the cutting tool and greatly reduces the efficiency of production.
Publication of the unexamined patent application Sho-52-127423 discloses a martensitic stainless steel excellent in machinability, including 0.003 to 0.40% of rare earth element. However, according to test result of the present inventors, the rare earth element has no effect to improve machinability and besides that it increases inclusions in the steel, particularly deteriorating the quality of threaded portion. In this steel, S (sulfur) is limited to 0.03% or less on the grounds that it impairs corrosion resistance and hot-workability. In addition, the hot-workability is merely evaluated based entirely on the condition of scabs created during rolling the steel into a plate, and it is not clear whether the hot-workability for forming a seamless steel pipe is sufficient or not.
Publication of the unexamined patent application Hei-5-43988 discloses a martensitic stainless steel including 13.0 to 17.0% of Cr, and optionally less than about 0.5% of S (preferably 0.1 to 0.5 to improve machinability). However, this steel includes about 1.5 to 4.0% of Cu. Since Cu is a component, which significantly deteriorates the hot-workability of steel, such a steel, including a large quantity of Cu, is not a suitable material for producing the seamless steel pipe by the inclined rolling method or the like.
Publication of the unexamined patent application Hei-9-143629 discloses an invention of a material pipe for steel pipe joint couplings, in which 0.005 to 0.050% of S is included as well as 5.0 to 20.0% of Cr so as to arrange "Mn/S" in 35 to 110. In this invention, the hot forging process is applied to produce the above material pipe for couplings, on the basis of the recognition that a Cr steel seamless pipe of high S content cannot be produced by the inclined rolling method such as the Mannesmann processes, due to its inferior hot-workability, That is, the material pipe disclosed in the publication is a short size pipe, which is produced by a hot forging process. In addition, while Al content is defined to 0.010 to 0.035% in the claim of the publication, actual Al content is unclear because there is no description of the Al content of the steel as examples. Since Al creates oxide compounds including Al.sub.2 O.sub.3, which is hard and has a high melting point, it accelerates wear on cutting tools, it is generally required to limit the Al content or to control the oxide composition by other components, such as Ca. However, these are not considered in the invention of this publication.
With respect to oil well pipes of 13Cr stainless steel (martensitic stainless steel), the API (American Petroleum Institute) Standards require "no scale on an inner surface of the pipe". In the 13Cr stainless steel, it is difficult to remove scale uniformly. Particularly a low sulfur martensitic stainless steel has a significantly low descaling property due to the high adhesion between the scale and the surface of the steel, and the scale is thereby apt to remain on the surface.